1. Field of the Invention
The invention relates to wood pulp bleaching liquors and their use in bleaching wood pulps.
2. Brief Description of the Prior Art
Hydrogen peroxide has long been used as an effective bleaching agent for paper products. It has found considerable use in the manufacture of bleached mechanical pulps. Hydrogen peroxide has also found some use in the final stages of a multiple stage bleaching sequence for chemical pulps as a method of inhibiting brightness reversion. There has been a small amount of work done with hydrogen peroxides as a semibleaching agent for chemical pulps; see for example Dugal, H. S., Ruhanen, M., 1981 TAPPI Environmental Conference, p. 93 and Lachenal, D., de Choudens, C., Soria, L., Monzie, P., 1982 International Pulp Bleaching Conference, p. 145. In all cases there has been little work completed on the effects of bleach liquor makeup in hydrogen peroxide bleaching.
Hydrogen peroxide bleaching is generally performed under alkaline conditions where it is most susceptible to decomposition. It is common practice to pretreat pulp furnishes with a chelating agent, such as diethylenetriaminepentaacetic acid (DTPA), to inhibit the decomposition. The chelating agent serves to tie up metal ions such as manganese, iron and copper which can catalyze the peroxide decomposition reaction. In addition, sodium silicate and magnesium sulfate are often used in the peroxide bleach liquor to stabilize the peroxide. When using hydrogen peroxide on chemical pulps, sodium silicate often does not provide the benefit that it does on mechanical pulps. Although the benefits of using sodium silicate and magnesium sulfate are widely recognized the mechanism of the benefit is not fully understood.
Very little work has been done investigating the effect that bleach liquor preparation may have on the bleaching process. The most extensive work has been completed by Burton (71st Annual CPPA Meeting (1985), p. A131)) where a wide variety of preparation procedures were examined. Burton also stated that the generally held belief is that a cascading system, for peroxide bleaching liquor preparation, is the best method for mixing peroxide bleach chemicals. The recommended order of addition in the cascading system (for mechanical pulps) was: magnesium sulfate, sodium silicate, caustic and lastly hydrogen peroxide.
We have found that if the bleach chemicals are mixed together in the following order:
MgSO.sub.4 : H.sub.2 O.sub.2 : and lastly NaOH PA1 MgCl.sub.2 : H.sub.2 O.sub.2 : NaOH
a significant increase in peroxide residual and final brightness is evidenced over the commonly recommended procedure of adding the peroxide last. We do not know what the beneficial mechanism is, but it may relate to a formation of magnesium/peroxide complex which would be inhibited if caustic is mixed with the magnesium prior to addition of peroxide.
Although the above-described procedure has not been recognized as providing a benefit in the literature or patents, Kutney (70th Annual CPPA Meeting (1984), p. B175) tested 75 stabilizing compounds including magnesium chloride (as a magnesium source) when bleaching groundwood or mechanical pulps. His experimental procedure indicates a mixing order of:
with additions of 0.22% and 0.49% magnesium. We typically use less magnesium. Kutney did not give any reason or benefit for using this mixing order and stated that magnesium does not have a beneficial effect on the bleaching process carried out by him. Additionally, he used it on groundwood or mechanical pulps whereas our advantage for its use is for a Kraft pulp.
Pero, R. W. and Dence, C. W., J. Wood Chem. Tech. 3(2): 195(1983) describe a procedure which may incorporate the same mechanism but is not the same process. They suggest steeping pulp in a suitable stabilization system, then adding peroxide and finally adjusting the pH with caustic. This is a multistep process whereas we are preparing a bleach liquor in a single step process.
The U.S. Pat. No. 3,023,140 describes a system for refiner semibleaching. In one example, peroxide and magnesium sulfate are mixed prior to caustic addition. However, this mixture is added to the refiner prior to adding caustic. This method also incorporates sodium silicate and is a sequential addition rather than a bleach liquor makeup. No comments as to the benefits of a particular chemical mixing order were made.